The present invention relates to a voltage measuring device, and more particularly, to a compact size voltage measuring device which can measure a wide range of voltage with a high accuracy.
To be an excellent voltage measuring device, it is desired that the voltage measuring device has a high input impedance, a high durability against high voltage, a high stability against temperature change, a high accuracy in displaying the measured voltage, and that it operates stably with low noise.
For obtaining the high input impedance, a vacuum tube has been widely used. However, the vacuum tube has disadvantages in that its performance is unstable and in that it produces high heat generated noise. Moreover, its size is very large. Recently, in place of the vacuum tube, a field effect transistor (FET) also having a high input impedance has become popular. When compared with the vacuum tube, the field effect transistor performs more stably with less heat noise, and can be assembled in a compact size. However, the voltage durability, or dielectric resistance, of the FET is so poor that it only can endure up to several 10's of volts under normal conditions.
In the field of electronics, many devices nowadays require a high voltage for their operation. For example, an electrophotographic copying machine requires several 100's to several 1000's of volts for its operation. For measuring such high voltage, an FET is an inappropriate element to be employed in the voltage measuring device.